Single sign-on is an attempt to address a problem that is common for all users and administrators. Various systems within the organization likely require the user to log on multiple times to multiple systems. Each one of these systems requires the user to remember a potentially different username and password combination. Most of us tire of trying to remember all this information and begin to look for shortcuts. The most common is to just write down the information. Walk around your office, and you might see that many of your co-workers have implemented the same practice. Single sign-on is designed to address this problem by permitting users to authenticate once to a single authentication authority and then access all other protected resources without reauthenticating. Before you run out and decide to implement single sign-on at your organization, you should be aware that it is expensive and if an attacker can gain entry, that person then has access to everything. Kerberos, SESAME, KryptoKnight (by IBM), and NetSP (a KryptoKnight derivative) are authentication server systems with operational modes that can implement single sign-on.
Thin clients can be considered a type of single sign-on system because the thin client holds no data. All information is stored in a centralized server. Thus, once a user is logged in, there is no reason for that user to reauthenticate.
Kerberos is a network authentication protocol created by the Massachusetts Institute of Technology (MIT) that uses secret-key cryptography. Kerberos has three parts: a client, server, and trusted third party (KDC) to mediate between them. Clients obtain tickets from the Kerberos Key Distribution Center (KDC), and they present these tickets to servers when connections are established. Kerberos tickets represent the client's credentials.
The KDC is a service that runs on a physically secure server. The KDC consists of two components:
The basic operation of Kerberos is as follows (and is shown in Figure 4.1):
Figure 4.1. Kerberos operation.
Some Kerberos literature uses the term principal instead of client. Principals can be a user, a process, or an application. Kerberos systems authenticate one principal to another.
Although Kerberos can provide authentication, integrity, and confidentiality, it's not without its weaknesses. One weakness is that Kerberos cannot guarantee availability. Some others are listed here:
The Secure European System and Applications in a Multivendor Environment (SESAME) project was developed to address some of the weaknesses found in Kerberos. SESAME incorporates MD5 and CRC32 hashing and two certificates. One of these certificates is used to provide authentication, as in Kerberos, and the second certificate is used to control the access privileges assigned to a client.
Access-control models can be divided into two distinct types: centralized and decentralized. Depending on the organization's environment and requirements, typically one methodology works better than the other.
Centralized Access Control
Centralized access-control systems maintain user IDs, rights, and permissions in one central location. RADIUS, TACACS, and DIAMETER are examples of centralized access-control systems. Characteristics of centralized systems include these:
Remote Authentication and Dial-In User Service (RADIUS) is a UDP-based client/server protocol defined in RFCs 2058 and 2059. RADIUS provides three services: authentication, authorization, and accounting. It facilitates centralized user administration and keeps all user profiles in one location that all remote services share. Although ISPs have used RADIUS for years, it has become a standard in may other ways. RADIUS is widely used for wireless LAN authentication. The IEEE designed EAP to easily integrate with RADIUS to authenticate wireless users. The wireless user takes on the role of the supplicant, and the access point serves as the client. If the organization has an existing RADIUS server that's being used for remote users, it can be put to use authenticating wireless users, too.
RADIUS functions are as follows (see Figure 4.2):
Figure 4.2. RADIUS authentication.
Other centralized authentication methods include TACACS and LDAP.
Terminal Access Controller Access Control System (TACACS) is available in three variations: TACACS, XTACACS (Extended TACACS), and TACACS+, which features two-factor authentication. TACACS also allows the division of the authentication, authorization, and auditing functions, which gives the administrator more control over its deployment. TACACS has failed to gain the popularity of RADIUS; it is now considered a somewhat dated protocol.
Decentralized Access Control
Decentralized access-control systems store user IDs, rights, and permissions in different locations throughout the network. Characteristics of decentralized systems include these:
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The CISSP Certification Exam
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